This invention relates to fiber reinforced preform sheets for compression molding and molded parts therefrom. More particularly, it relates to such sheets formed from chips of substantially parallel filaments coated with thermoplastic resin.
Fiber reinforced thermoplastic composite preform sheets for compression flow molding applications are well known in the art. During processing, the preform sheets are heated to a temperature which melts the matrix resin and placed in a compression molding tool. The tool surface is held below the matrix resin melt temperature. The tool is rapidly closed and pressure applied to flow the material and fill out the mold. Pressure is maintained for a specified dwell time to solidify the matrix resin, after which the finished part is ejected. Typically, the preform sheet used in such applications is sized to cover between 40 and 90% of the mold surface area, the remainder to be filled by flow during the compression cycle.
Known methods for making flow moldable composite preforms include layering alternate sheets of random reinforcing materials with polymer films followed by heating and cooling under pressure to form an integrated solid sheet, extrusion of molten polymer onto sheets of random reinforcing materials followed by heating and cooling under pressure to form an integrated solid sheet, and blending of polymeric powders and latexes with reinforcing fibers using a paper making process to form a nonwoven mat which can be further heated and cooled under pressure to form a solid integrated sheet.
This invention is concerned with composite preform sheets which are intended for heating prior to molding by radiant or contact means such that the bulk thermal conductivity of the sheet is an important factor for efficient processing. Such preform sheets generally take the form of well consolidated, low void content sheet structures. Consolidation under pressure is required to overcome the lofting force of the reinforcing fibers, to wet the fibers with matrix resin, and to remove air and voids in the structure which hinder thermal conductivity.
When consolidated preform sheets of the known prior art are reheated prior to molding, the composite sheet will reloft again to typically 2 to 10 times its compressed thickness as the matrix resin approaches and passes its melt temperature. This lofting results in a substantial loss of thermal conductivity during the heating process (due to the incorporation of voids) causing loss of heating efficiency and requiring longer duration heating. Often the time/temperature cycle leads to thermal degradation of the matrix polymer.